A fluidized bed combustion unit generally comprises a bed of particulate material to which air is passed by one of a number of arrangements (for example by an array of sparge tubes extending generally horizontally through the material of the bed) to fluidize the material of the bed and support combustion of fuel fed to the bed. The bed material may be formed of the material to be combusted or be formed of an inert particulate material (for example sand) to which air and fuel is passed. The air passed to the bed to fluidize the bed supports combustion of fuel passed to the bed and may also support combustion of volatile gases generated within the bed which may continue to burn in the volume above the bed (second phase combustion).
Fluidized bed combustion units may be used as primary heat generators (a hot gas generator or boiler for example) as gas producer plant (in which the bed is operated endothermically as a gas producer) or as a dryer (in which there is substantially complete combustion within the bed).
The fluidizing air fed to the bed may be augmented with an inert gas (for example recycled flue gas) in order to ease control of the operation of the bed.
A fluidized bed combustion unit when used as a hot gas generator is normally enclosed within a reaction vessel completely lined with refractory materials and provided with no means for cooling the above bed, secondary phase combustion zone. As a result when such a hot gas generator is shut-down from a full working load condition, particularly if it is a near instantaneous, trip or crash shut-down, difficulties result from heat lock within the vessel and the effects of heat radiation from the refractory lining material back onto the bed.
In addition to the heat lock and back radiation difficulties the fluidized bed normally contains relatively high proportion of hydrocarbon fuel (for example coal) which at full load operating conditions may be as much as 15% by weight of the total bed weight. The fuel in the bed may contribute to the magnitude of the heat contained within the bed particularly when there is a slippage of air into the bed which slippage may be promoted, for example, by an induced draft fan in the plant served by the hot gas generator. Where such air slippage occurs the air promotes low level, but significant, combustion of the hydrocarbon fuel in the bed causing the bed temperature to rise.
In a relatively short space of time this increase in bed temperature may lead to clinkering of the bed material which it will be appreciated is now substantially static.